1. Field of the Invention
The present invention relates to a sensorless brushless DC motor (hereinafter referred to as sensorless BLDC motor) used where the temperature or humidity is high or a position sensor is difficult to establish, and more particularly, to an apparatus and method for controlling a sensorless BLDC motor which effectively filters out the influence of noise.
2. Related Art
The electronic industry has been investigating brushless DC motors because of the small size of such motors. In a brushless motor, it is essential to exactly measure the position of a motor rotor to exactly supply the voltage and a position measuring apparatus can be used for this purpose. However, when a sensor of the rotor position measuring apparatus does not operate properly due to high humidity or temperature, or, in an apparatus, such as a compressor, in which it is impossible to employ such a position measuring apparatus, it becomes difficult to exactly measure the rotor position. Moreover, because the space occupied by the rotor position measuring apparatus in the BLDC motor is so large, a sensorless BLDC motor is desirable.
Such a sensorless BLDC motor controls the power supplied to a stator coil based upon an induced voltage at the stator coil terminal. In a sensorless BLDC motor, the rotor has no position sensor.
An edge detection signal is generated by detecting the edge of the waveform of a comparative output which is obtained by comparing an induced voltage generated at each terminal of a plurality of stator coils located along with the rotational direction of the stator with a standard electric potential (0 V).
Presently, in order to remove a variation in the output caused by a spike voltage Vs produced when the power supply is turned on or off, the compared output is standardized. This standardized compared output is used to control power supplied to each stator coil, i.e., commutation is performed based on this output.
However, in instances where the edge signal is generated at a point at which the power of the stator coils is not turned on or off, the system will not operate properly. In other words, when commutation time of the stator coil is inaccurate, the motor is not rotated smoothly.
In order to solve the problem discussed above, improved motors have been developed. The motor disclosed in Japanese Patent Laid-open No. 4-317585 (1992), which is one of the improved motors, will now be considered.
FIG. 1 is a block diagram illustrating a control circuit of a conventional sensorless BLDC motor while FIG. 2 is a waveform illustrating the operating states of the units of FIG. 1. Referring to FIGS. 1 and 2, stator coils Lu, Lv, and Lw are arranged according to the rotational direction of the rotor, and are connected to a commutation signal generating circuit 1. The control circuit of FIG. 1 also includes a multiple phase driving circuit 2, a zero level comparison circuit 3, and an edge detection circuit 4. In addition, the control circuit includes a timing control circuit 5, an edge detection control circuit 6, and a starting oscillation circuit 8 as well as a clock converting or switching circuit 8 and a control unit 9.
The commutation signal generating circuit 1 generates the multiple phase control signal for controlling the power supply to each stator coil Lu, Lv and Lw, i.e., the commutation signals, according to a signal clock CK. The multiple phase driving circuit 2 supplies the power to drive stator coils Lu, Lv and Lw according to the phase based upon the commutation signals. The zero level comparison circuit 3 compares the induced voltages U-N, V-N and W-N generated at each terminal of the stator coils Lu, Lv and Lw with a standard electrical potential 0 V. A hysteresis comparison circuit having a predetermined input value is preferably used as the zero level comparison circuit 3. The edge detection circuit 4 detects the edge of the output wave forms of the comparison circuit 3 according to their phase, and logically adds an edge detection pulse of each phase, thereby generating a standard clock signal CK2 of the commutation signal generating circuit 1. The timing control circuit 5 delays the standard clock signal CK2 generated by the edge detection circuit 4 as much as a predetermined time t2, and transmits a delayed standard clock signal CK used in generating the commutation signals of the commutation signal generating circuit 1.
The edge detection control circuit 6, which includes a delay circuit 61 and an OR logic gate 62, prevents the edge detection of the waveforms of the compared outputs of each corresponding phase when the power supplied to the stator coils Lu, Lv and Lw is turned on or off, based upon a status signal indicating the operating state of the commutation signal generating circuit 1. The oscillation circuit 7 generates a standard clock signal CK1 when the motor starts. At this time, the switching or converting circuit 8 receives the standard clock signal CK1 from the oscillation circuit 7, and transmits the standard clock signal CK to the commutation signal generating circuit 1 through the timing control circuit 5. After the motor is energized, i.e., driven, the switching circuit 8 receives the standard clock signal CK2 from the edge detection circuit 4, and transmits the standard clock signal CK to the commutation signal generating circuit 1 through the timing control circuit 5. The control unit 9 controls the switching circuit 8 according to the standard clock CK2 outputted from the edge detection circuit 4.
Considering the operation of the control circuit of FIG. 1, the driving circuit of the sensorless BLDC motor derives the standard clock signal CK used for controlling commutation by comparing the induced voltages U-N, V-N and W-N generated at each terminal of the stator coils Lu, Lv, and Lw with each standard electrical potential 0 V, and by detecting each edge of the waveforms of the compared outputs according to their phases. At this time, the influence of the spike voltage Vs generated by switching of the power supply of each stator coil Lu, Lv, and Lw is detected by the comparison circuit 3. Before being detected by the edge detection circuit 4, the spike is selectively precluded from detection by the edge detection control circuit 6. However, depending upon the control state, when the phase voltage is measured in a transition state or when there is a heavy motor load, noise that interferes with edge detection can be produced even at points where the power supplied to the stator coils is not turned on or off. Moreover, the prior art motors allow the induced voltages to exceed the zero voltage level while the edge detection is prohibited and disturbs the precision of the commutation signals.